I recently got the 2x model joining the 1.4x I got earlier and thus have the complete set. Following are some observations on them in general, specifically regarding the Canon mount models but likely to be similar on other mounts.

Both are similar in physical appearance, except the 2x is about double the length to accommodate the extra elements inside. The front element of both protrudes a bit from the mount, so they will be limited in which lenses they will fit. Mechanically speaking, pure longer focal length lenses are ok as they don't tend to have anything at the mount to get in the way. This is not a problem as it doesn't make so much sense to use these on wide angle lenses.

The only Sigma lens I have that is officially compatible with these is the 150mm macro. Using it with the 1.4x doesn't seem to affect image quality in macro use, but longer focusing distance shots seem softer wide open. I haven't used it with the 2x enough yet for a comparison. Due to the effective light loss at close focusing, the AF range is limited further out when used with the 1.4x. AF is not supported with the 2x, even at distant focus where it should in theory work. Even the tape trick doesn't help, as the seek operations fail to lock.

They also fit on the Canon 100-400L. Again this combination is MF only. I found it in practice hard to get accurate MF, and the sharpness was noticeably degraded with the 1.4x so I wouldn't say this was worth doing normally. I can't imagine the 2x being any better.

Both also fit the TS-E 24mm. Normally this wouldn't make much sense, but in this case I do find 24mm rather wide for my typical uses. As the lens is MF, you don't lose out by using the TC. The lens isn't the sharpest anyway, although the reduction in effective aperture might be a bit much.

So given the above, why did I get them at all? As said, the 1.4x does seem to work well on the macro lens at close focus. The increased magnification is useful for macro work, where MF is typically used so the loss from adding the TC is not significant. Although I've yet to use it as such, I'm hoping the 2x will do similarly for that bit more magnification.

One final observation. The 2x TC has quite some clearance on the back end, so you can mount the 1.4x behind it, stacking them for 2.8x. This is not recommended by Sigma, but functionally it works. I've not tested it seriously for quality but I wouldn't get my hopes up.

Thanks popo for the report.
I'm quite astonished that the 2x TC does not work with the 150/2.8 Sigma as the effective aperture still is f/5.6 which should AF ok, shouldn't it?
What is the "tape trick" you mentioned?

If you have time it would be interesting to compare two images one with converter and the other w/o but upscaled to have the same resolution. When I did this comparison once with a Kenko 1.5x TC, the up-rezzed version was looking even better to my eye (see here).

I don't know why it doesn't work with that lens. But the Sigma compatibility list says it is MF with 2x, and that seems to be the case in practice.

The "tape trick" is covering some contacts of the teleconverter so it doesn't report its presence. The system then doesn't know it is there and will try to AF even if it shouldn't work. I haven't come across a case were this was worth doing yet.

Here's part one of the results. I want to do two tests - one of "distant" subjects, and one for close up. This may be more about the lens than teleconverter though.

Originally I pointed it out of a window for some distant targets. Snag is the air was too wobbly so I gave up on that.

Inside I set up the Sigma box as a target, which was just over 2m away as indicated on the focus scale. This is less than 1:10 magnification. I manually focused using live view and used a wireless remote release to take the shots.

Normally I'd align the output by system f value, but as the lens quality seems limiting wide open, getting better as it is stopped down even a little where it seems to remain.

Images below are lossless crops from camera generated jpeg. Sharpness was set to 0 for these shots.

Lens by itself at f/2.8, f/4, f/5.6, f/8

Lens with 1.4x teleconverter at system f/4, f/5.6, f/8

Lens with 2x teleconverter at system f/5.6, f/8

Lens with both teleconverters at system f/8

To avoid the glow wide open, here is the upscaling comparison using the one stop down images for reference:

1.4x upscaled image, 1.4x teleconverter image

2x upscaled image, 2x teleconverter image

To me, at this specific physical aperture size (keeping the lens quality the same), both teleconverters give better sharpness compared to upscaling. Comparing results at a constant system f value would be harder as you'd have to dodge the lens characteristics wide open and also the sensor diffraction limit at the other end.

I intend to follow this up at a later time with a macro level comparison, as that is how I will mostly be using these parts normally.

Thanks for the comparison.
It would be fairer methinks, if you use the same effective aperture for the up-converted images.
It would also help to have a target with finer structures/smaller print black on white. This makes it easier to judge contrast/sharpness.

To dig up this old thread, first I'll respond to Thomas' last comment which I failed to previously. The reason I didn't compare equal system apertures was because the lens is noticeably softer wide open. By keeping the lens one stop down from max with and without converter, I'm keeping the lens intrinsic performance constant and not confusing the teleconverter's performance with lens performance with varying aperture setting.

Anyway, why dig up this old thread? I saw some pigeons sitting on my garden fence while I was cooking, so took some shots handheld with the 100-400L. They were a little far, so I thought I'd give the 2x converter another try. Handholding this was a messy as I remember, as MF was rather difficult. Even the smallest turn on the focus ring shifted it a lot, and it was hard to see clearly in the viewfinder. I put it on a tripod, and to my surprise when I switched to live view I found the contrast AF in that mode worked!

Why leave IS on with the tripod? At such a massive focal length, the tripod was introducing a slight wobble which I noticed the IS could compensate for.

I wont post any of the 400mm 100% crops here as the AF wasn't "good enough" and they're actually softer than this! By traditional "big picture" viewing assumptions, the DoF from various calculators come out around 10cm. Because of the high pixel density and 100% viewing, in practice it is probably much less than that.

The main take away point here is that with contrast based AF working, it is possible to get accurate focus especially given the thin DoF. Assuming contrast AF usage and adequate light given the small apertures, it is quite usable.

I'll probably have a sigma 2x converter soon, I can get it secondhand for 100 euros, which is a decent price, even if the sharpness turns out to be poor. 400mm F/5.6 handheld still sounds doable, and I have a monopod if it doesn't work out.

_________________I take pictures so quickly, my highschool was "Continuous High".

I went Sigma as it matched the macro lens they are primarily used with.

On the Tamron, did you get a good chance to compare? If so, what were the differences you experienced? Also, which Tamron converter? If I'm not mistaken they have several designs from low cost 4 element, to a "pro" version with 7 elements (or was that the 2x?), and possible more versions in between.